Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on centers of gravity, centers of buoyancy, the metacenters of vessels, and on how these interact.
Ballast is dense material used as a weight to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship, submarine, or other floating structure that holds water is called a ballast tank. Water should be moved in and out from the ballast tank to balance the ship. In a vessel that travels on the water, the ballast will be kept below the water level, to counteract the effects of weight above the water level. The ballast may be redistributed in the vessel or disposed of altogether to change its effects on the movement of the vessel.
The deck department is an organisational team on board naval and merchant ships. Seafarers in the deck department work a variety of jobs on a ship or vessel, but primarily they will carry out the navigation of a vessel from the bridge. However, they are usually also responsible for supervising and monitoring any maritime cargo on board, as well as ensuring maintenance of the deck and upper hull structure, monitoring the stability of the ship, including loading and discharging ballast water, carrying out mooring operations, and finally anchoring a ship.
Among the merchant fleets of countries that have ratified the STCW Convention, the department and its manning requirements, including the responsibilities of each rank are regulated by that STCW Convention. The department is led by deck officers, who are licensed mariners, and they are commanded overall by the ship's captain.
A bilge keel is a nautical device used to reduce a ship's tendency to roll. Bilge keels are employed in pairs, one for each side of the ship. A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase hydrodynamic resistance, making the ship roll less. Bilge keels are passive stability systems.
On commercial shipping the bilge keel is in the form of a strake, or small keel or blister, running along much of the length of the hull. They are typically fitted one on each side, low down on the side of the hull, so as not to increase the draft of the vessel. In battleships they were often quite large and used as part of the torpedo protection system.